C++ partial template template specialization

Question

I want to pass a partial template specialization to template template parameter but I'm getting an error. Im not sure why excatly this doesent work.

template<template<typename, int> class V, typename T, int N, int... Indexes>
class Swizzle
{
    // ...
};

template<typename T, int N>
struct Vector;

template<typename T>
struct Vector<T, 3>
{
    // ...

    union
    {
        // ...
        Swizzle<Vector, T, 3, 0, 0, 0> xxx;
    };
};

Error:

'Vector': invalid template argument for template parameter 'V', expected a class template 'Swizzle': use of class template requires template argument list

Problem appears only on MSVC

What compiler and flags are you using? Is this code sufficient to repro the problem? This works for me [on Coliru](http://coliru.stacked-crooked.com/a/0fefaa7c92d8cc73) once I add some semicolons. — metal, Feb 04 '20 at 14:04
Im using VS2019 and fixing the missing semicolon's, still produces this error. I tried pasting the rest of my code into godbolt using clang as compiler, that works so i guess this must be a VS2019 error?. — sorte33, Feb 04 '20 at 14:16
offtopic: `union` in C++ code is not a best idea. Note there is `std::variant` (c++17) which is modern version of the `union`. — Marek R, Feb 04 '20 at 14:39

Yakk - Adam Nevraumont · Accepted Answer · 2020-02-04T14:56:37.893

3

Within the class template Vector, Vector refers to both the type of this instance of the template and the template itself.

This should compile:

template<class X, int M>
using Self = Vector<X,M>;
// ...

union
{
    // ...
    Swizzle<Self, T, 3, 0, 0, 0> xxx;
};

I suspect MSVC is wrong here, but am uncertain.

edited Feb 04 '20 at 14:56

answered Feb 04 '20 at 14:40

Yakk - Adam Nevraumont

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fails on clang and gcc https://godbolt.org/z/Rox8M- but type definition in global scope works: https://godbolt.org/z/NyKfLr – Marek R Feb 04 '20 at 14:51
@MarekR Fixed. Just a name collision. – Yakk - Adam Nevraumont Feb 04 '20 at 14:56

Jarod42 · Answer 2 · 2020-02-04T15:11:11.007

Within the class with class name injection, Vector might refer to both the type of this instance of the template and the template itself.

In the following cases, the injected-class-name is treated as a template-name of the class template itself:

[..]

it is used as a template argument that corresponds to a template template parameter

So Msvc is incorrect here.

Possible workaround:

Swizzle<::Vector, T, 3, 0, 0, 0> xxx;

Demo

C++ partial template template specialization

2 Answers2